1. Field of the Invention
The present invention relates to hydraulic dredge apparatus. More particularly, the present invention relates to an apparatus for maintaining the vacuum level of a hydraulic dredge substantially at but just below the point of dredge pump cavitation.
2. General Background
In the hydraulic dredging industry today, the vacuum level is maintained below the point of dredge pump cavitation by increasing power to the pump as the vacuum level drops and decreasing power to the pump as the vacuum levels rises. The amount of power supplied to the dredge pump is controlled by an operator who watches a display of the vacuum level. This method of operation typically results in a vacuum level as shown in FIG. 3 herein.
There are a number of U.S. patents related to hydraulic dredging, including the patents which are listed below, all of which are hereby incorporated by reference.
Fonnesbeck U.S. Pat. No. 3,111,778, discloses a control device for a hydraulic dredge which measures the output pressure of the dredge pump to determine what amount of water to allow to enter the intake pipe. It uses control cables to open and close the water inlet valve.
Ehrenberg U.S. Pat. No. 2,938,536, discloses a valve which shuts down a pipeline near a pump when the pressure on either side of the pump or the flow of liquid through the pipeline adjacent the pump exceeds a predetermined point.
Hofer U.S. Pat. No. 2,603,234, discloses a butterfly relief valve in a hydraulic dredge pipe which automatically opens when there is excess vacuum in the pipe.
Hofer U.S. Pat. No. 3,263,615, discloses a similar relief valve which opens when the vacuum levels at two longitudinally spaced locations in the dredge pipe are equal.
Hoffman U.S. Pat. No. 3,119,344, discloses a hydraulic dredge apparatus which includes means for expelling gas from the dredge pipe.
Frazier U.S. Pat. No. 3,448,691, discloses a dredge apparatus in which a jet pump is used to inject water into a pipe having an intake at one end and a dredge pump at the other end.
Hadjidakis U.S. Pat. No. 3,514,881, discloses an apparatus for adjusting the section slot in a drag suction dredge by vertically adjusting the lower end of the suction pipe.
De Koning U.S. Pat. No. 3,772,805, discloses a dredge in which the lower end of the intake pipe is moved up and down to attempt to control the amount of sand being dredged. A relief valve opens to allow water into the intake pipe if danger of excessive cavitation arises.
Sandberg U.S. Pat. No. 4,278,365, discloses a hydraulic dredge in which solids concentration is controlled by raising and lowering the suction nozzle and increasing and reducing the flow rate of the pump.
Beck U.S. Pat. No. 4,444,209, discloses a system for removing water from a slurry in order to increase the concentration of the slurry. Air is admitted into a chamber as needed to help prevent cavitation.